Diffusion of oxygen in Mg-doped α-Al2O3: The corundum conundrum explained

It has been a puzzle for over two decades that the enhancement of oxygen diffusion in alpha-Al2O3, with respect to the amount of Mg doping, is several orders of magnitude less than expected. The standard model, which envisages that transport is mediated by oxygen vacancies induced to compensate the charge of Mg2+ ions substituting Al(3+ )ions, has not been able to explain this anomaly. Here, we report a detailed study of populations of point defects and defect clusters in Mg-doped alpha-Al2O3. By taking into account calculated defect formation energies from the literature, the condition of charge neutrality, and the environmental parameters (chemical potentials) under which the anomalous trend in oxygen diffusivities were previously observed, we are able to arrive at an explanation. A nonlinear relationship between Mg concentration in the system and key native point defects, which serve as mediators of self-diffusion in alpha-Al2O3, is predicted: The concentrations of such defects increase much more slowly in the supersaturation regime than in the presaturation regime, matching the anomalous result previously observed in alpha-Al2O3. We identify the reason for this as buffering by positively charged Mg interstitials and Mg-oxygen vacancy clusters, which compensate the negative charges of Mg substitutional defects (Mg-Al(1-)). This study answers part of the long-standing question about self-diffusion in alumina, referred to by Heuer and Lagerlof in 1999 as the corundum conundrum.

Automated summary

The puzzle of why the enhancement of oxygen diffusion in alpha-Al2O3 is much less than expected with an increase in Mg doping has been perplexing for over two decades. In this study, the researchers conducted a detailed investigation of point defects and defect clusters in Mg-doped alpha-Al2O3. By considering defect formation energies, charge neutrality, and environmental parameters, they were able to explain the anomalous trend in oxygen diffusivities. Their findings revealed a nonlinear relationship between Mg concentration and key native point defects, which explains the previously observed anomaly in oxygen diffusion in alpha-Al2O3.